The present invention relates to electro-optics and more particularly to confocal measuring using two wavelengths of light to determine the optimal position and displacement of a movable object.
Typically, in order to determine the position of an object, an optical fiber cable connects a light-source and a light detector to a sensor. An optical signal generated by the source is transmitted through the cable to the sensor. The sensor, in response to a physical variable that is desired to be measured, such as displacement, modulates a characteristic of the optical signal in accordance with changes in the physical variable. The modulated signal is thereafter transmitted to the detector which converts that signal to a useful output representative of the magnitude of the physical variable.
It is known to utilize a portion of the modulated signal as a feedback control signal for insuring a constant level output from the light-source. However, this device still does not overcome the problems that may arise from instabilities in the light detectors or in the optical cables. Generally, the modulated signal is divided and transmitted simultaneously through at least two optical cables to respective measurement detectors coupled to each cable. This arrangement requires that these two (or more) optical cables have matching optical properties and performances, so that accurate measurements can be derived from the modulated signal produced by the sensor. The use of multiple parallel routes will increase the sources of drift caused because of instabilities and changes in the operation of the light-source or detector.
With a conventional microscope, the image is blurred when not in focus. In contrast, with a confocal microscope, an object which is not in focus appears very dim and blurred with minimal contrast. Thus, using a confocal microscope, a strong output is only produced when the object is in focus. Confocal microscopes are well known in the art. It is also known in the art to use single-mode optical fibers in confocal microscopes and to use the same fiber for transmitting and detecting the reflected confocal signal. An example is described by R. Juskaitis and T. Wilson in their article entitled xe2x80x98Direct-View Fiber-Optic Confocal Microscope,xe2x80x99 published in Optics Letters, Volume 19, Number 22, November 1994. R. H. Webb and F. J. Rogomentich in their article entitled xe2x80x98Microlaser Microscope using Self-Detection for Confocality,xe2x80x99 published in Optics Letters, Volume 20, Number 6, March 1995, describe a scanning confocal microscope using its own source lasers as detectors and a beam splitter and a single avalanche photodiode (APD) to detect the reflected light.
Reference is now made to FIG. 1, which illustrates a prior art confocal scanning unit, generally designated 10, for maintaining a target object 12 in focus. The prior art confocal device 10 is operative to move either the optical head mechanism or the object 12 in the z-plane in order to maintain the object in focus. The confocal scanning unit 10 comprises a radiation source 14, a first lens system 16 and a second lens system 18. Confocal scanning unit 10 further comprises a beam splitter 20 and a third lens system 22.
Rays 24a and 24b travel from radiation source 14 via first lens system 16 and second lens system 18 to object 12. Rays 24a and 24b are then reflected as rays 26a and 26b, respectively, via second lens system 18, beam splitter 20 and third lens system 22 to a detection unit 28 via an aperture 25.
The detected signal strength as a function of the axial displacement between the optical assembly and the target object using the prior art confocal measuring device of FIG. 1 is graphically illustrated in FIG. 2. The amplitude of the detected signal (y-axis) as a function of the axial displacement (x-axis) is shown as the object is scanned into and out of focus. In this example, the signal 30 shows an axial response having a full width half maximum (FWHM) of approximately 3 xcexcm. If the object is not in focus, the reflected signal will be less than the signal associated with the zero displacement line 32. However, a major disadvantage with this system is that when reading the signal value while the unit is not in focus, it is not possible to determine the direction of the offset of the object, i.e., whether it is closer or farther away from the zero displacement line 32.
Dutch Patent No. NL 9001202 assigned to Phillips N V describes a confocal scanning unit using a single source of radiation together with a beam splitter. This unit uses additional lens and/or beam splitters and/or detectors to determine the displacement of the scanned unit. Such a unit is bulky, complicated and expensive to produce.
It is also known to use two wavelengths of light to determine the position of a movable element. U.S. Pat. No. 4,596,925, issued to Gilby, describes a fiber optic displacement sensor, which transmits two different wavelengths via an optical fiber to a filter. The filter and the movable element of the sensor cooperate with each other to modulate the intensity of the first beam in accordance with the position of the movable element thereby transforming the first beam into a measurement beam. The second beam and its resulting reference beam are used to compensate for the effects that the optical paths have on the intensities of the first beam and its resulting measurement beam.
U.S. Pat. No. 4,946,275, issued to Bartholomew, describes a distance measurement system for monitoring changes in distances between a source of illumination and a reflective surface. A collimated beam of light from a white light source passes through a grating to split the beam into a spectrum which is directed to the reflective surface at an unknown distance therefrom. The dispersed light bounced off the reflective surface enters a receiver fiber optic device connected to a detector for determining the distance between the grating and the reflective surface.
U.S. Pat. No. 5,196,866, issued to Ferschl et al., teaches an imaging apparatus utilizing a rotating carrier member having a plurality of laser diodes and a plurality of optical fibers connecting the laser diodes to a movable writing head. A focusing arrangement is provided for focusing the writing beam with respect to the writing element and comprises a laser diode for generating a focusing beam of light projected onto the writing element and a photocell. The focusing beam and writing beams are physically separated at the writing head.
U.S. Pat. No. 5,257,038, issued to Ferschl et al., teaches a focusing device for focusing a light source which generates a first beam of light of a wavelength selected to be actinic with respect to the writing element. The focusing device includes a focusing laser diode mounted on and movable with a movable write head to minimize noise in the focusing signal.
None of the above mentioned prior art references which use two wavelengths or white light can be used for finding the optimal position of an object.
The present invention is a confocal optical system that utilizes fiber optic components in its construction. The system comprises a light source, two detection units, an aperture and an optical element all optically coupled to a fiber optic coupler via optical fibers. In addition, the present invention also comprises a novel automatic focusing device which utilizes chromatic aberration to maintain a target object in optimal focus. The device comprises two light sources having different wavelengths of light, an optical element, an aperture, two detection units and a beam splitter. One light source is used to achieve initial focus and to illuminate the target object. The second light source is used to maintain the target object in optical focus. Light reflected off the target object is measured by one of the detection units. The magnitude of the light of the second wavelength measured by the detection unit is utilized to maintain focus. A second detection unit is utilized to measure the intensities of the two light sources to yield normalized results. Further, the present invention also comprises a method of diagnosing an optical system. The method comprises comparing a current set of readings of light against a set of reference readings taken at an earlier point in time. If the difference exceeds a predetermined threshold, a problem is indicated.
The present invention utilizes the novel technique of accurately determining the displacement of an object utilizing chromatic aberration. Chromatic aberration causes the lens to focus different colors of light at different points. That is, two light sources having different wavelengths, but located at substantially the same point, have images in different planes. Thus, by using two different light sources within an automatic focusing device to scan an object, it is possible to determine whether the scanned object is in focus and furthermore, the direction in which the object is out of focus, i.e., nearer or farther away.
One advantage of using such a technique is that the same channel can incorporate both the beam used for the actual writing of data onto the media and the beam used for focusing. For example, even when the writing beam is off, the focus beam can be maintained. A second advantage is that the detected signals give an indication of in what direction to move in order to correct the out of focus condition. This is in contrast with typical confocal systems which do not have such an automatic focus mechanism.
There is therefore provided in accordance with the present invention a confocal optical device illuminating a target object, comprising a light source for generating a beam of light, an optical element for illuminating the target object, a first detection unit for measuring the magnitude of light reflected off the target object, the magnitude of light being related to the focal position of the target object, an optical coupler optically coupled to the light source, the optical element and the first detection unit, an aperture operatively situated such that the reflected light causes a maximum signal to be generated by the first detection unit when the target object is in focus, optical connection means for optically connecting the light source, the first detection unit and the optical element to the optical coupler, and wherein a maximum signal measured by the first detection unit indicates that the target object is in proper focus.
The optical device further comprises a second detection unit for measuring the intensity of the light source, the second detection unit optically connected to the optical coupler, the intensity of the light source utilized to normalize the magnitude measured by the first detection unit in determining the proper focus distance for the target object. The optical connection means comprises a plurality of optical fibers.
Further, there is provided in accordance with the present invention an automatic focusing device for maintaining a target object at an optimum focal distance, comprising a first light source for generating a beam of light having a first wavelength, a second light source for generating a beam of light having a second wavelength, an optical element for illuminating the target object, a first detection unit for measuring the magnitude of light of the first wavelength reflected off the target object, the first detection unit for measuring the magnitude of light of the second wavelength reflected off the target object, both magnitudes of light being related to the focal position of the target object, an aperture operatively situated such that light of the first wavelength causes a maximum signal to be generated by the first detection unit when the target object is in focus, a beam splitter for coupling light from the first and second light sources to the target object, the beam splitter coupling light from the target object to the first detection unit, wherein the magnitude of light of the second wavelength as measured by the first detection unit is related to the focal distance of the target object, and wherein the direction of the swing of the magnitude of light of the second wavelength is indicative of the direction of misfocus of the target object.
The optical device further comprises a second detection unit for measuring the intensity of the first light source, the beam splitter coupling light from the first light source to the second detection unit, the intensity of the first light source utilized to normalize the magnitude of light of the first wavelength measured by the first detection unit in determining the proper focus distance for the target object.
The optical device further comprises a second detection unit for measuring the intensity of the second light source, the beam splitter coupling light from the second light source to the second detection unit, the intensity of the second light source utilized to normalize the magnitude of light of the second wavelength measured by the first detection unit in determining the proper focus distance for the target object. The optical element comprises at least one lens.
In addition, the first detection unit comprises a spectrally sensitive beam splitter for splitting light in accordance with its wavelength, a first detector for sensing a magnitude of light of the first wavelength, and a second detector for sensing a magnitude of light of the second wavelength.
Also, there is provided in accordance with the present invention an automatic focusing device for maintaining a target object at an optimum focal distance, comprising a first light source for generating a beam of light having a first wavelength, a second light source for generating a beam of light having a second wavelength, an optical element for illuminating the target object, a first detection unit for measuring the magnitude of light of the first wavelength reflected off the target object, the first detection unit for measuring the magnitude of light of the second wavelength reflected off the target object, both magnitudes of light being related to the focal position of the target object, an aperture operatively situated such that light of the first wavelength causes a maximum signal to be generated by the first detection unit when the target object is in focus, optical coupling means optically coupled to the first light source, the second light source, the optical element and the first detection unit, optical connection means for optically connecting the first light source, the second light source, the first detection unit and the optical element to the optical coupling means, wherein the magnitude of light of the second wavelength as measured by the first detection unit is related to the focal distance of the target object, and wherein the direction of the swing of the magnitude of light of the second wavelength is indicative of the direction of misfocus of the target object.
The optical device further comprises a second detection unit for measuring the intensity of the first light source, the beam splitter coupling light from the first light source to the second detection unit, the intensity of the first light source utilized to normalize the magnitude of light of the first wavelength measured by the first detection unit in determining the proper focus distance for the target object.
The optical device further comprises a second detection unit for measuring the intensity of the second light source, the beam splitter coupling light from the first second light source to the second detection unit, the intensity of the second light source utilized to normalize the magnitude of light of the second wavelength measured by the first detection unit in determining the proper focus distance for the target object.
The optical coupling means comprises at least one fiber optic coupler. The first detection unit comprises a spectrally sensitive beam splitter for splitting light in accordance with its wavelength, a first detector for sensing a magnitude of light of the first wavelength, and a second detector for sensing a magnitude of light of the second wavelength.
Still further, there is provided in accordance with the present invention, in an automatic focus system having a first light source of a first wavelength, a second light source of a second wavelength, a method of achieving optical focus distance for a target object, the method comprising the steps of illuminating the target object with the first light source, stepping through a focusing range for the optical system, determining the maximum signal of a light beam of the first wavelength reflected off the target object for the entire focusing range, the maximum signal representing optical focal distance for the target object, storing the maximum signal, illuminating the target object with the second light source, measuring a signal D3 of a light beam of the second wavelength reflected off the target object, the signal D3 representing optical focal distance for the target object, measuring, on a substantially continuous basis, a signal D4 of a light beam of the second wavelength reflected off the target object, calculating the difference xcex94D2=D4xe2x88x92D3, and adjusting the focal distance to the target object in accordance with the sign and magnitude of xcex94D2.
The method further comprises the steps of measuring a signal S2 corresponding to the intensity of the second light source, and normalizing the signals D4 and D3 utilizing the signal S2 before the step of calculating.
There is also provided in accordance with the present invention, in an optical system for illuminating a target object and having a light source, a method for performing diagnostics on the optical system, the method comprising the steps of stepping through a focusing range for the optical system, determining a maximum magnitude D of the light reflected off the target object for the entire focusing range, measuring the intensity S of the light source, at a later time: stepping through a focusing range for the optical system, determining a maximum magnitude Dxe2x80x2 of the light reflected off the target object for the entire focusing range, measuring the intensity Sxe2x80x2 of the light source, calculating the quantity       "LeftBracketingBar"                  D        S            -                        D          xe2x80x2                          S          xe2x80x2                      "RightBracketingBar"    ,
and indicating to the system that a problem exists if the quantity exceeds a predetermined threshold.